In Iowa, an engagement ring given in contemplation of marriage is an impliedly conditional gift.  However, it is a completed gift only upon marriage.  The Iowa courts have held that if a wedding is called off, for whatever reason, the gift of the engagement ring is not capable of becoming a completed gift and must be returned to the donor.[1]

[1] Fierro v. Hoel, 465 N.W.2d 669 (Iowa Ct. App. 1990)